The high speed operation of conventional air driven contra angle handpieces tends to accelerate bearing wear and gives rise to a need for bearing replacement at periodic intervals. Because of the small size of the parts and the precision with which they must be fitted together, bearing replacement for such dental handpieces is still commonly a factory rebuilding operation requiring the skill of highly trained workers. Such reconstruction is, unfortunately, relatively expensive and time consuming. Since downtime for a given handpiece depends on the availability of trained workers capable of carrying out such reconstruction, and since the number of such skilled workers is limited, it is not uncommon for dentists to wait days or even weeks for factory rebuilding of their handpieces.
Efforts have been made to simplify the rebuilding of such handpieces so that dentists might perform the work themselves, but such efforts have been only marginally successful. A common approach has been to form a rigid cartridge consisting essentially of a rotor (which includes a bur tube), a pair of bearing assemblies, and a shell which securely locks the bearings in alignment and at the proper pre-load. When replacement becomes necessary, the dentist removes the cap from the handpiece housing, withdraws the old cartridge, and replaces it with a new self-contained cartridge unit. U.S. Pat. Nos. disclosing such a cartridge arrangement are 3,255,527, 2,945,299, 3,324,553, 3,084,439, and 3,411,212.
Among the disadvantages of such a rigid cartridge system are the increased bulk caused by the secondary shell which locks the bearings and other components in proper operating relation and the requirement that the cartridge be manufactured to extremely close tolerances so that it will fit precisely in existing housings. The result is that a system utilizing a rigid cartridge construction to permit field replacement is achieved only at relatively high cost and with some disadvantages of increased bulk and weight.
To avoid the disadvantages of rigid cartridge systems, some manufacturers have marketed replacement rotor subassemblies in which all of the replacement parts are carried by spindles which only support the parts until they are mounted within a handpiece housing. While such subassemblies are sometimes called "cartridges", they differ from other cartridge systems because, among other things, no shells are utilized for rigidly securing the parts together in fixed relation. The bulk and weight arising from the use of a cartridge shell are therefore avoided; however, such a system does increase the possibility that some of the components of the replacement assembly, such as the resilient support rings for the bearings, may be shifted out of position, become damaged (the resilient rings may be pinched or cut by malassembly), or fail to seat properly within a handpiece housing unless considerable care is exercised in making the replacement.
An object of this invention is therefore to overcome the aforementioned disadvantages of prior constructions. Specifically, it is an object to provide a system which permits quick and simple field reconstruction of a worn handpiece at relatively low cost and with virtually no risk of misalignment or faulty assembly of the parts.
In the system of the present invention, no rigid cartridge is used. The combination of parts which might for convenience be referred to as a "cartridge " is really a subassembly frictionally held together by resilient rings which serve the ultimate purpose of cushioning the parts and reducing sound development in a fully assembled handpiece. Such resilient rings are protected against displacement during field replacement by protective surrounding elements and by the remaining components of the rotor cartridge replacement assembly. More specifically, the lower bearing assembly and the resilient ring which encircles it are surrounded by a flexible plastic cup which is adapted to seat within the lower end of the head housing of a dental handpiece. The flexible side wall of the plastic cup has a slight taper which not only facilitates insertion into the handpiece housing but also insures a secure interfit between the cup and housing. In other words, the wall of the plastic cup is capable of flexing inwardly to a limited extent to conform to the inside dimensions of the head housing.
Such flexure of the cup's side wall is facilitated by a multiplicity of notches which are spaced circumferentially about the rim of the cup and which define a multiplicity of spring fingers formed integrally with the cup's side wall. The spring fingers surround and retain the resilient support ring for the lower bearing assembly and, especially when the rotor cartridge or subassembly is mounted within the housing of a handpiece, such fingers exert a compressive force on the ring which serves to hold the cup, ring, and lower bearing assembly (along with other elements such as, for example, a wave washer) frictionally together. The security of such frictional engagement is increased by reason of the fact that the resilient ring tends to bulge or expand slightly into the notches of the cup, thereby reducing the possibility of independent relative movement of the parts.
In addition to the above advantages, the flexible plastic cup also serves as a non-metallic spacer between the lower bearing assembly and the handpiece housing and, along with the resilient support ring, further reduces the transmission of sound generated during operation of the high speed handpiece.
The rotor cartridge or subassembly is part of a larger assembly which includes a housing cap and a support tool for holding the parts together during shipment and storage and for inserting and securing such parts in place when handpiece reconstruction is required. The tool includes an enlarged upper retaining element in the form of a wrench which is keyed to the threaded handpiece cap so that, following insertion of the rotor subassembly into the cavity of the handpiece housing, the wrench may be rotated to secure the threaded cap in place. The tool also includes a spindle which extends through both the cap and the rotor cartridge, and a lower retaining element in the form of a sleeve which is located at the lower end of the spindle and which holds the cap and cartridge in place upon that spindle until such time as removal is desired. One of the retaining elements, either the wrench or sleeve, is removable from the spindle to permit extraction of the spindle after the replacement rotor assembly has been secured within the handpiece housing. In the embodiment disclosed, the removable member is the lower retaining element which takes the form of a plastic sleeve frictionally secured to the lower end of the spindle.
Other features, objects, and advantages of the system of this invention will become apparent from the specification and drawings.